This application is based upon and claims the benefit of priority of the prior Japanese Patent Applications No. Hei 7-339556 filed on Dec. 26, 1995 and No. Hei 8-108311 filed on Apr. 26, 1996, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a brake control apparatus mounted on a vehicle and more particularly to a brake control apparatus which can efficiently decelerate the vehicle.
2. Description of Related Art
A conventional brake control apparatus mounted on a vehicle is normally composed of a brake pedal controlled by a driver, a master cylinder which generates master cylinder pressure in response to the control of the brake pedal, a brake conduit for transmitting this master cylinder pressure, wheel cylinders for receiving the transmitted master cylinder pressure to generate braking force in wheels, and others. The master cylinder pressure is increased/reduced in such a brake control apparatus in response to driver""s depressing force to the brake pedal.
In the conventional brake control apparatus, however, the wheel cylinder pressure drops in synchronism with the drop of the master cylinder pressure when the driver weakens the depressing force to the brake pedal. For instance, if the driver weakens the depressing force because the driver is unable to withstand reaction force of the brake pedal caused by increase of the master cylinder pressure when the driver is stepping on the brake pedal hard in order to obtain large braking force, the wheel cylinder pressure drops concurrently with decrease of the depressing force. In such a case, there is a possibility of prolonging an actual stopping distance more than a braking distance which the driver has expected in the beginning of the braking because the braking force may become insufficient against the will of the driver.
It is desirable for the vehicle brake control apparatus to exhibit a high braking performance in compliance to the braking performance which the driver has expected in the beginning of the braking when the driver has started to control the brake pedal.
Accordingly, it is an object of the present invention to provide a brake control apparatus which can exhibit a braking performance in compliance with a driver""s request in the beginning of the braking of a vehicle.
According to the present invention, a characteristic changing device is adopted in a brake control apparatus for a vehicle. That is, a differential pressure is set by the characteristic changing device such that first brake hydraulic pressure which generates braking force for the wheels is higher than second brake hydraulic pressure which is generated by a hydraulic pressure generating source when the second brake hydraulic pressure in the hydraulic pressure generating source is reduced. Due to the differential pressure set as described above, the first brake hydraulic pressure which exhibits enough braking force is kept applying to a wheel braking force generation member even if the second brake hydraulic pressure is reduced in the hydraulic pressure generating source. Thereby, the braking performance expected by a driver in the beginning of the braking of the vehicle is satisfied.
It is also possible to differentiate the characteristic of change of reduction of the second brake hydraulic pressure from that of the first brake hydraulic pressure when the second brake hydraulic pressure is reduced in the hydraulic pressure generating source, while equalizing a change of increase of the second brake hydraulic pressure with that of the first brake hydraulic pressure. That is, the first brake hydraulic pressure is increased with the same response with the pressure-increase in the hydraulic pressure generating source, so that the braking force is assured. In contrast, the braking performance can be satisfied by delaying or attenuating the change of reduction of the first brake hydraulic pressure than the second brake hydraulic pressure when the second brake hydraulic pressure of the hydraulic pressure generating source is reduced.
Further, when the second brake hydraulic pressure in the hydraulic pressure generating source or the first brake hydraulic pressure applied to the wheel braking force generation member becomes less than a predetermined value, it is desirable to equalize the first brake hydraulic pressure with the second brake hydraulic pressure by the characteristic changing device. In this case, the first brake hydraulic pressure may be zeroed at least when the second brake hydraulic pressure in the hydraulic pressure generating source is zeroed. Accordingly, it is possible to inhibit the braking force from being generated reliably when there is no driver""s request to apply the braking force to the vehicle.
That is, even if a brake operating member is returned by a driver, the degree of reduction of the first brake hydraulic pressure is lessened for a predetermined period of time, not reducing the first brake hydraulic pressure applied to the wheel braking force generation member as it is, and the degree of the reduction is increased thereafter. Thereby, similarly to the case described above, a braking distance may be shortened while maintaining the high deceleration of the vehicle even if the brake operating member is loosened unintentionally in panic or the like.
When the brake operating member is controlled so as to reduce the second brake hydraulic pressure in the hydraulic pressure generating source, it is also possible to substantially keep the first brake hydraulic pressure by the characteristic changing device for a predetermined period of time. As a result, a braking distance may be shortened while maintaining the high deceleration of the vehicle even if the brake operating member is loosened unintentionally in panic or the like.
When the brake operating member is operated so as to reduce the second brake hydraulic pressure in the hydraulic pressure generating source, the characteristic changing device may execute a change of reduction of the first brake hydraulic pressure applied to the wheel braking force generation member with a delay of time larger than a change of increase of the first brake hydraulic pressure applied to the wheel braking force generation member when the brake operating member is operated so as to increase the second brake hydraulic pressure in the hydraulic pressure generating source.
That is, in the state that the brake operating member is returned, the reduction of the first brake hydraulic pressure is delayed. Thereby, the braking distance may be shortened while maintaining the high deceleration of the vehicle even if the brake operating member is loosened unintentionally in panic or the like.
It is noted that the brake operating member described above is not limited to a brake pedal controlled by the driver. Rather, it also includes a control member of an automatic brake system and the like.
When the driver controls the brake operating member so as to reduce the second brake hydraulic pressure, it is possible to execute a reduction of the first brake hydraulic pressure applied to the wheel braking force generation member with a first pressure-reduction gradient in the initial stage of a pressure reduction operation of the brake operating member. After the initial stage of the pressure reduction operation of the brake operating member, it is possible to execute a reduction of the first brake hydraulic pressure applied to the wheel braking force generation member with a second pressure-reduction gradient which is greater than the first pressure-reduction gradient.
That is, when the brake operating member is returned by the driver, the degree of reduction of the first brake hydraulic pressure is lessened in the initial stage of the pressure-reduction operation of the brake operating member and the degree of reduction is raised thereafter. It also allows the braking distance to be shortened while maintaining the high deceleration of the vehicle even if the brake operating member is loosened unintentionally in panic or the like.
When the driver executes a pressure-reduction operation on the brake operating member, it is possible to cause the characteristic changing device to keep the first brake hydraulic pressure applied to the wheel braking force generation member at a pressure of more than a predetermined value. It also allows the braking distance to be shortened while maintaining the high deceleration of the vehicle even if the brake operating member is loosened unintentionally in panic or the like.
It is noted that a brake booster whose boosting rate is variably controlled may be adopted as the characteristic changing device.
It is also possible to change the characteristic of pressure-reduction by the characteristic changing device only when the deceleration of the vehicle is greater than a predetermined value. This is because it is possible to determine that the reaction force from the brake operating member is large, even if the braking operation is in a panic mode or the like in which a large brake hydraulic pressure needs to be assured when the deceleration of the vehicle is large.
When a brake pedal is used as the brake operating member, it is also possible to change the characteristic of pressure-reduction only when the pedal stroke of the brake pedal is greater than a predetermined value. This is because it is possible to determine that the reaction force from the brake pedal is large, despite it is the braking operation being in a panic mode or the like in which a large brake hydraulic pressure needs to be assured when the pedal stroke is large.
It is also possible to change the characteristic of pressure-reduction by the characteristic changing device only when the depressing force to the brake pedal is greater than a predetermined value. This is because it is possible to determine that the reaction force from the brake pedal is large, despite it is the braking operation being in a panic mode or the like in which a large brake hydraulic pressure needs to be assured when the depressing force to the pedal is large.
It is also possible to change the characteristic of pressure-reduction only when master cylinder pressure is greater than a predetermined value. This is because it is possible to determine that the reaction force from the brake pedal is large, despite it is the braking operation being in a panic mode or the like in which a large brake hydraulic pressure needs to be assured when the master cylinder pressure is large.
It is also possible to change the characteristic of pressure-reduction only when wheel cylinder pressure is greater than a predetermined value. This is because it is possible to determine that the reaction force from the brake pedal is large, despite it is the braking operation being in a panic mode or the like in which a large brake hydraulic pressure needs to be assured when the wheel cylinder pressure is large.
It is also possible to change the characteristic of pressure-reduction only when a wheel deceleration is greater than a predetermined value. This is because it is possible to determine that the reaction force from the brake pedal is large, despite it is the braking operation being in a panic mode or the like in which a large brake hydraulic pressure needs to be assured when the wheel deceleration is large.
It is also possible to change the characteristic of pressure-reduction only when anti-skid control is performed. This is because it is possible to determine that the reaction force from the brake pedal is large during the anti-skid control even though the braking operation being in a panic mode or the like in which a large brake hydraulic pressure needs to be assured.
When the driver executes a pressure-reduction operation to the brake operating member so as to reduce the brake hydraulic pressure, the first brake hydraulic pressure applied to the wheel braking force generation member may be increased by a brake hydraulic pressure increasing device.
That is, the first brake hydraulic pressure is increased once when the brake operating member is returned by the driver. It allows the braking distance to be shortened while maintaining the high deceleration of the vehicle even if the brake operating member is loosened unintentionally in panic or the like.
It is noted that a system such as a traction control system (TRC) may be used to increase the first brake hydraulic pressure Further, the braking distance may be shortened further by increasing the first brake hydraulic pressure until when the brake operating member is completely returned.
Further, according to another preferred embodiment of the present invention, the vehicle is braked by providing braking force to the wheels by the wheel braking force generation member based on the second brake hydraulic pressure generated in the hydraulic pressure generating source in response to the operation of the brake operating member. Then, when the brake operating member is operated so as to reduce the second brake hydraulic pressure, the variation of reduction of the first brake hydraulic pressure applied to the wheel braking force generation member is lessened with respect to the variation of the operation of the brake operating member by the characteristic changing device.
That is, even if the brake operating member is being returned, the degree of reduction of the first brake hydraulic pressure is lessened, not reducing the first brake hydraulic pressure applied to the wheel braking force generation member as it is. It allows the necessary brake hydraulic pressure to be assured even if the brake operating member is loosened unintentionally in panic or the like. Therefore, the braking distance may be shortened while maintaining the high vehicle deceleration.
According to an another preferred embodiment, the vehicle is braked by providing braking force to the wheels by the wheel braking force generation member by receiving the second brake hydraulic pressure generated in the hydraulic pressure generating source in response to the operation of the brake operating member. Then, when the brake operating member is operated so as to reduce the second brake hydraulic pressure, a gradient of reduction of the first brake hydraulic pressure applied to the wheel braking force generation member from the beginning of the pressure-reduction until an elapse of a predetermined time is made smaller than that after the elapse of the predetermined time.